Printed wiring boards (PWBs) consist of non-conducting substrates, such as fiberglass/epoxy or polyimide, on which conducting circuits are deposited and discrete passive elements may be mounted. Multichip modules (MCMs) are devices that consist of a collection of integrated circuit (IC) chips or die that are mounted on a high-density interconnect substrate, such as alumina, with a high utilization ratio of die area to substrate area (up to 90-95%). The advantages of MCM technology over conventional packaging schemes, such as printed wiring boards, include very high packaging densities, clock speeds in excess of 500 MHz, lower power due to reduced capacitance, higher reliability, and better thermal matching of the die to the substrate material. The current disadvantages include the high cost of substrates, limited availability of bare die, and difficulties in testing at high clock speeds.
An important requirement for virtually every PWB and MCM application today is extremely high reliability. A demanding series of accelerated life tests, including moisture resistance, salt atmosphere, thermal shock, thermal cycle, constant temperature bake, lead integrity, and fine and gross leak tests, are usually performed on assembled PWBs and MCMs. Since semiconductor die are the major building blocks of an MCM, the first focus of yield, testability, and diagnostic evaluation must be at the die level (KGC concept of a "known good die"). However, performance analysis must be done for each product level (die, MCM, and system) and for each test condition.
A substantial number of different approaches are used by various vendors in the field. For example, MCMs are often classified according to the substrate technology (e.g. MCM-L, -C, and -D) which determines the possible interconnect density. The complexity of MCM design is reflected in the accepted definitions of the types of modules being offered to electronics manufacturers.
The use of a wide variety of processes has resulted in a lack of standardization which complicates the selection of substrates, choice of interconnection, and passive element integration technology. Thus, passive components (e.g., resistors, capacitors, inductors) are often attached discretely to the previous level of assembly. The insertion of low-profile passive components into PWBs and MCMs is costly, laborious and time consuming. Processes compatible with existing technologies which are able to incorporate the fabrication of passive elements into existing technologies are needed.
In hybrid and MCM technologies, passive components are produced by thin films (&lt;5 .mu.m), thick films (&gt;10 .mu.m) or may be discretely attached to the substrate, e.g. chip-resistors and chip-capacitors. Discrete passive components are less desirable because of laborious insertion of the components into the MCM circuit, and their use is limited to special conditions where very low or high values are needed.
Thin film technology is based on the deposition of passive elements by electron-beam evaporation or sputtering. Thin film resistors may be nickel-chromium, tantalum nitride, or silicon carbide films; and dielectrics include silicon monoxide, silicon dioxide, and tantalum oxide. In thick film technology, inks or pastes are screen printed to the ceramic substrate. The resistive components of resistor pastes may comprise: ruthenium oxide, thallium oxide, indium oxide, mixtures of precious metals or tungsten-tungsten carbide. Dielectric materials are largely based on the ferroelectric ceramic barium titanate with various additives and glass-ceramic mixtures.
Electronically conducting polymers have often been categorized as non-processable and difficult to manage or manipulate, because of their insolubility in the conducting form. Only recently has it been shown that polymers such as polyaniline can be dissolved using functionalized sulfonic acids. For polypyrrole, this can be achieved by using its derivatives e.g., poly (3-octylpyrrole)! which are known to be soluble in different solvents, or by treatment in dilute aqueous sodium hypochlorite solutions, ammonia or mono-, di- or tri-substituted amine (co)solvents. Another method of solubilizing polypyrrole is the process of polypyrrole chain deprotonation in basic solutions, which causes a transformation of conducting polypyrrole into a non-conducting polymer of quinoid structure.
The lack of processability of conducting polymer materials, e.g., solution or melt processing, infusability and poor mechanical properties, e.g., ductility, have slowed down their emerging commercial applications. While electrochemical preparation of conducting polymers has been shown to be the most satisfactory process from the viewpoint of fundamental investigations, it is likely to be inappropriate for the large-scale industrial production of bulk quantities of these materials. This is particularly true where large molecular entities, e.g., copolymers or different additives, need to be incorporated into conducting polymer matrices in order to obtain tailored performance characteristics.
Consequently, there remains a need for improved methods of manufacturing passive electronic elements in both printed wiring boards and multichip modules. It would be desirable to have a conducting polymer that avoids polymer solubility problems, can easily incorporate additives to obtain desirable characteristics such as flexibility and photoinitiation, minimizes hazardous chemicals, requires fewer process steps to apply/deposit/coat a workpiece, facilitates increased conductor densities on a substrate surface, and allows for passive electronic elements to be formed directly on the substrate. Additionally, it may be desirable to have a conducting polymer that can easily incorporate additives to obtain the characteristic of photoinitiation. Furthermore, it would be desirable to have a method of manufacturing printed wiring boards and multichip modules having increased interconnect densities, improved manufacturability and improved product quality.